This invention relates to a method and apparatus for tire pressure determination particularly, but not exclusively, applicable to automotive vehicle tire pressure determination, notably as applied to automobile and truck tire pressure determination, but applicable to vehicle tire pressures generally.
Tire pressure surveys and other available data indicate a considerable need for some more convenient and readily used method for tire pressure determination than the conventional method of directly measuring the pneumatic pressure in the tire at the time of inflation and subsequently.
If some external sensor system could be provided which did not require any active intervention on the part of the vehicle driver, this would obviously be extremely beneficial and useful. However, although systems for remote sensing of vehicle operating parameters have been proposed including the use of ultrasonic sensors, no system of that kind appears to be readily applicable to tire pressure determination on a remote basis.
Nethertheless, we are aware from our co-pending European patent application EP 96307897.7 (our reference P52835EP), due to be published May 28, 1997, that sensory determinations through the tires of a vehicle can be effectively made using piezo electric sensors, notably such in cable and similar format whereby the loading of the cable is effected in such a manner that the load is applied at spaced apart positions on the piezo electric material.
We are aware of course that piezo electric cable type materials have been used widely for sensing traffic movements, with the piezo electric material in the format of a road-surface-mounted device which produces an electrical impulse on the passage of a vehicle. Such impulses may have been analysed in various ways in the past, mainly simply on the basis of mere counting of pulses. Possibly proposals may have been made for more sophisticated analysis of such data.
The present invention is based upon our discovery that the passage of a vehicle tire over a piezo electric cable produces a waveform which has characteristics of shape and form which enable the pressure of the tire to be determined by appropriate analysis of such form and/or shape. It is believed that it has not previously been proposed to analyse the waveform and/or shape of such piezo-electric-cable-produced electronic pulses for this purpose.
Moreover, we have discovered that the pulses thus produced are affected in terms of their pulse width and height by the speed of the vehicle and its weight. An important aspect of the present invention relates to the interpretation of the data obtainable from the piezo electric device to take account of these factors, including a method for actually eliminating or offsetting their effects.
The present invention takes as the state of the art from which claim 1 commences as the use of signals from piezo electric cable devices mounted on roadway surfaces for numerical traffic monitoring purposes.
There is disclosure in:
EP-A-0 545 641 (Exxon)
EP-A-0 656 269 (Exxon)
of a system for determining pneumatic tire pressure and/or velocity in which an two-dimensional array of force sensors in a driveway or the like determine the pattern of force distribution exerted by a tire in its contact footprint across the width of the tire and a computer determines tire pressure and/or velocity from the sensed contact forces.
There is disclosed in U.S. Pat. No. 4,630,470 (Brooke) a system for determining tire pressure of vehicles as they pass an instrumented check point on a roadway. Rigid corrugations on the roadway set the tires into vibration with a waveform which is a function of tire pressure. Below the roadway surface directly beneath a metal plate on which the corrugations are formed to provide the road surface there are provided multiple costly transducers which convert the mechanical vibrations caused by the tires to identical electrical waveforms. The spacing 26 between adjacent ridges of the corrugations or ribs varies with the type of vehicle being checked. It has been found that the ideal spacing for a jeep is 2.0 inches and 3.1 inches for a 5 tonne military truck. In accordance with the invention, the spacing between the ridges is variable within this range. The waveforms produced by the tires passing over these ribs are subjected to spectral analysis based on the concept of detecting one or more tires which has a differing spectrum from the others and therefore is at a different pressure. No means is disclosed for actual quantitative numerical determination of the tire pressure, but only for detecting differences in pressure.
To the best of the Applicants"" knowledge there has been no prior proposal for the use of a piezoelectric cable sensor/detector device located on a roadway traversed by automotive vehicles to provide a signal which is generated by direct loading of the device by the compressive engagement by the vehicle tires across the lateral widths of the tires without any mechanical intervention or intermediary device (such as the ribs of the Brook patent), and from which signal the tire pressure is determined quantitatively directly by non-spectral analysis and without the need for comparison with detectors responding to other tires of the same vehicle.
There is disclosed in EP-A-0 387 092 (Gebert) traffic monitoring equipment such as traffic speed detection equipment. The invention is concerned with the provision of validation checks for such equipment to enhance the reliability and accuracy and convenience of operating such equipment. Such validation is needed, for example, in order to check the level of insulation resistance between conductors in a particular cable. The invention is applicable to any traffic data collection equipment. The idea is to monitor the gradual or abrupt deterioration of a cable in service conditions leading to degrading of the insulation resistance which can lead to erroneous measurements and degraded accuracy or reliability. The invention provides apparatus for validation checks applied to traffic monitoring equipment. For example, the invention checks the level of insulation resistance of the equipment (page 2, line 43). It also validates operation by means of a facility for signal strength monitoring (page 3, line 5). For this purpose, a minimal signal level is required as a valid trigger for time pulses and (line 12) xe2x80x9cin reference to a curve of the signal drawn on a time base the first pulse (be it positive or negative), must have a minimum steepness ie a minimum value of the first differential of magnitude of the pulse with respect to timexe2x80x9d. Claim 2 refers to the curve of the signal needing to have a minimum steepness for use for traffic speed detection purposes. Accordingly, the voltage profile has relevance only in relation to achieving a xe2x80x9cminimal signal level before being detected as a valid trigger for time pulsesxe2x80x9d (see page 3 at line 7). The shape of the waveform is only used to determine whether the waveform is or is not acceptable in terms of meeting the prescribed requirement for a minimal signal. Having achieved that minimal signal level, the waveform is used merely as a pulse and speed is determined as stated on page 4 at line 32:xe2x80x94xe2x80x9cin normal speed timing equipment using such cables two parallel spaced cables are set on the road in an array and the time is measured between a pulse being generated in a first cable and a pulse generated in the second cable by the same wheel set of a vehicle. On this basis speed is calculated by the formula distance divided by timexe2x80x9d. This disclosure provides no suggestion that the skilled person could expect to determine numerically speed or any other vehicle parameter by means of waveform analysis.
There is disclosed in EP-A-0 502 803 (ECM) equipment for establishing efficient measurement of the dynamic loads applied to a roadway by traffic. It is stated that currently known signal processing methods do not take into account various factors including the effect of under-inflated tires and the object of the invention is to eliminate all of the adjustments necessary with previous proposals.
FIG. 3 of the EP 803 specification shows the form of signal xe2x80x9caccording to the tire inflationxe2x80x9d. In the description it is stated that FIGS. 3a and b demonstrate the form of the signals received for two identical axles with respectively inflated and uninflated tires 3a and b. The measurement show that at constant speed and loads when the tire is uninflated the duration of the signal is greater and the peak value is weaker. The description goes on to say that these two diagrams show that measurement (of vehicle dynamic loads applied to the roadway) by amplitude is not correctly founded and that it is not sufficient to take into consideration the duration of the pulse in order to perform a speed correction, since the duration of this pulse is dependent on the inflation of the tire.
By inspection of FIGS. 3a and 3b of the EP 803 specification it is plain that the disclosure amounts to this. With a properly inflated tire you obtain a fairly sharp and upright voltage pulse 23 as shown in FIG. 3a and with an uninflated tire you get a voltage pulse having a slightly lower amplitude and a significantly greater pulse width or wavelength. Technically, the foot print of the tire on the road is larger when the inflation pressure is lower and the vehicle load is applied over this larger area to the roadway surface as shown at 25 in FIG. 3b. The result is a reduced voltage pulse height or amplitude and an increased voltage pulse duration while the larger foot print passes over the sensor. No disclosure relating to analysis of the shape or profile of the waveform is provided.
According to the invention there is provided a method and apparatus for automotive vehicle information determination including tire pressure and related determinations, as defined in the accompanying claims.
In an embodiment described below, there is provided a method and apparatus whereby tire pressure determination is effected by the simple expedient of causing a vehicle wheel/tire assembly to pass over an elongated piezo electric cable sensor element for compressive engagement with it across the lateral width of the tire, the sensor being located on or at the roadway surface or the like. This can be done at relatively low speed, for example during manoeuvres within an automotive servicing facility, or in a garage or petrol station forecourt.
As the vehicle tire in question passes over the cable, so as to compress and load same across the full lateral width of the tire (ie the tire""s entire surface area at the relevant portion of its circumference), the cable produces an electromotive force comprising positive and negative-going waveform voltage elements, as shown in FIGS. 4 to 6 of the accompanying drawings. These waveforms, we have discovered, possess characteristic shape components which enable pressure determination to be made on the basis of calibrated reference to previously obtained data. The shape of the voltage profiles obtained depend upon the tire pressure and hence the latter can be readily determined on a look-up reference basis.
In the embodiments, account is taken (by means of a software algorithm) of the influence of vehicle weight and speed (though generally low) in order to isolate as far as possible the data available providing a direct measure of tire pressure. Vehicle weight has a direct relationship to the amplitude of the voltage pulse produced, due to the fact that the voltage produced by a piezo electric device has a direct relationship to the mechanical load applied to the device. Accordingly the weight component of the waveform produced by the device is processed on the basis of calibration data relating vehicle weight to voltage produced, whereby in a given test the output can be standardised by reference to a predetermined and uniform vehicle weight.
As regards the effect of vehicle speed, a direct approach to eliminating this effect is available by merely repeating a given test with the vehicle passing successively over spaced sensor elements at a known separation, whereby the speed component of the data can be readily calculated and eliminated or reduced by an adjustment process.
Therefore with respect to eliminating or reducing the effect of vehicle speed on the waveform, the following details of the approach can be adopted in a simple and cost-effective manner, namely to adopt the use of twin or paired piezo electric cable sensor elements at a known spacing and position so that the tire in question passes over each, successively. In this way, there are generated corresponding twin voltage peaks at time intervals corresponding to vehicle speed. By a simple calculation technique the vehicle speed can be determined and the effect of the vehicle""s exact speed can be deduced or offset by reference to known data relating to vehicle speed. In this way, there is obtained a waveform or voltage profile for analysis which, as far as can reasonable be achieved, is free of the effect of vehicle speed. A corresponding adjustment can be made as described above, in respect of vehicle weight.
By arranging for the piezo electric cables to extend non-transversely with respect to the vehicle travel direction it can be arranged that individual wheel assemblies or axles at opposite sides of the vehicle produce corresponding individual signals which can be very easily related to the corresponding tires so that remedial pressure-adjustment steps can be corresponding easily taken.
In the described embodiment the elongated piezo electric cables are mounted at the roadway surface so as to project at least a small distance above the surface while being partially located within corresponding grooves or channels formed at the roadway surface. Below each piezo electric sensors is a base plate which serves as a stable reference surface to support the sensor while it is being loaded in use. Above each sensor is a flexible protective cover extending lengthwise of the sensor to enable it to be loaded by the vehicle wheels while being resistant to surface wear and abrasion and damage by impact with vehicle tires. For example, the protective cover may be convex in format and formed from a flexible polymeric material such as natural or synthetic rubber with or without suitable fillers and/or embedded reinforcement materials. Likewise, various plastics sheet materials and metallic sheet materials will have the requisite wear and flexibility characteristics. Considerable variation in the details of mounting the piezo electric cables may be provided, from the simplicity of merely laying a (sheathed or otherwise protected) cable on a hard surface in the path of vehicles, through a variation of this in which the cables 26 and 28 in FIG. 2 hereof are simply supported on the roadway surface instead grooves therein, through to a more deeply grooved arrangement in which the cables are mainly below the roadway surface but have a flexible material above them in the groove and projecting from the groove for engagement by the vehicle tires.
We are aware that proposals may have been made for interpretation of data obtainable from road-mounted vehicle sensors by reference to the gradient of the voltage profile at some particular presumably symmetrically located position thereon, and the present application is not concerned with such an approach which is considered to be a somewhat subjective basis for analysis as compared with the approach adopted by the present invention, namely that of pressure determination by reference to the overall shape and profile of the voltage pulses produced.
Attention is also drawn to the fact that the embodiments of the present invention produce their corresponding analyzable waveform pulses by the direct compression or loading of the elongated piezo electric cable across the lateral widths of the vehicle tires as they traverse it. The tires themselves exert directly on the simply cable sensor (but of course through one or more protective layers whereby the sensor is not unacceptably damaged or worn by each vehicle passage) a loading which varies according to the tire pressure in a determinable quantitative manner. There is no need to cause the tire to actuate a vibration-generating corrugated plate with multiple transducers producing a signal requiring spectral analysis as in the Brooke reference, nor is there any need to analyse the pattern of sensed contact forces in the tire footprint as in the Exxon references.
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings which show apparatus and corresponding voltage waveform data obtained from the apparatus according to the invention, the waveform data illustrating the differences in waveform profiles and shape produced at tire pressures below and at and above the recommended pressures.